It is commonly known that insulating oil, such as mineral oil, is used in power transformers. A number of methods and treatments are known to treat insulating oil and to avoid the contamination thereof. Specifically oxygen and water can contaminate the insulating oil at the free surface in the expansion vessel and cause oxidation and humidification. Usually the insulating oil has a free surface to avoid pressure injuries or the like, since a high pressure should be avoided even when the insulating oil is heated by the power transformer. Many power transformers up to today are so called free breathers, which means that the free surface of the insulating oil in the expansion vessel is exposed to the atmosphere and therefore to particles, oxygen and water. In a free breather power transformer there is no gas tight barrier between the gas that comes into contact with the free surface of the insulating fluid and the atmosphere. The gas is thus normally air. However, it is common practice to have an in-line cartridge filled with some drying agent, such as silica gel, as a moisture barrier against the ambient air.
It is known in the prior art to use some kind of diaphragm, which is arranged to be in fluid communication with the free surface of the insulating oil in the conservator. The diaphragm is used to isolate the gas in contact with the free surface of the insulating oil from the atmosphere and specifically from oxygen and water. The diaphragm has a low permeability to oxygen and water.
GB 945,688 discloses an apparatus permitting a liquid contained in a reservoir, such as an expansion vessel, to expand and contract freely without coming into contact with the outside atmosphere and thus without the risk of humidification and/or oxidation by that outside atmosphere. The reservoir contains an inert gas in communication with a diaphragm container exposed to the outside atmosphere and comprising resilient means. The resilient means are used to provide a pressure of the inert gas slightly below and slightly above the atmospheric pressure when the level or volume of the liquid varies. GB 945,688 further discloses to install a dryer in the path of the inert gas to permanently dry the inert gas, since the volume comprising the inert gas is not completely air tight, allowing oxygen, water or vapour to enter the volume comprising the inert gas and eventually contaminate the insulating oil.
The apparatus of GB 945,688 is expensive to install and it is not modular, thus it cannot be extended, for example in case the transformer is replaced by a bigger transformer. Further, the apparatus must be fixedly installed with a power transformer during assembly/production of it;—Retrofitting the apparatus of GB 945,688 to an existing transformer is not easy and it may involve high cost. In addition the apparatus of GB 945,688 does not illustrate the use of a back pressure device or the like for enhancing the safety of the system.
Other known solutions comprise so called continuous degasser devices, which draw oil directly from the main tank of the transformer or reactor and continuously degas said oil. Such solutions, if they work as intended, have the side effect that the interpretation of dissolved gas analysis becomes very difficult. Additionally continuous degassers are expensive, require regular maintenance and many continuous degassers do not reduce the oxygen to an acceptable level, at least not when considering the cost of installing and maintaining them.
JP 2006 295017 discloses a conservator which makes a moisture absorbent such as silica gel unnecessary. The conservator comprises an oil storage which is arranged in communication with an oil-containing electric device containing insulating oil sealed therein, and stores the insulating oil flowing in/from the oil-containing electric device, and a gas storage which stores gas to isolate the gas hermetically from the fresh air. The gas storage is capable of changing its shape in response to the flow of the gas caused by the inflation/shrinkage of the insulating oil. The gas storage has a bag-like body or a sheet-like body that changes its shape to store the flowing gas.